This invention relates to a solventless ultraviolet radiation-curable protective coating composition. More particularly, it relates to a silicone coating composition which contains no water or organic solvent and, when applied to a substrate, may be quickly cured by exposure to UV radiation to form a protective, abrasion-resistant coating thereon.
Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles such as trains, buses, taxis and airplanes. Lenses for eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant, transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering and lighter than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and/or ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and often requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature and good dimensional stability. It is also self-extinguishing, and it is easily fabricated. Acrylics, such as polymethylmethacrylate, are also widely used transparent plastics for glazing.
Attempts have been made to improve the abrasion resistance of transparent plastics. For example, scratch-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225; 3,986,997, 3,976,497 and 4,027,073, for example, describe such compositions.
In commonly-assigned copending U.S. application Ser. No. 964,910, filed Nov. 30, 1978, coating compositions having improved resistance to moisture and humidity and ultraviolet light are disclosed. It was discovered therein that, in direct contrast to the teachings of U.S. Pat. No. 3,986,997, compositions having a basic pH, i.e., 7.1-7.8, do not immediately gel but in fact provide excellent abrasion-resistant coatings on solid substrates.
Recent discoveries offer a significant advantage over many of the heretofore known silicone coating compositions in that they do not require heat in order to initiate the cure reaction. Instead, ultraviolet radiation is employed, expending considerably less thermal energy than conventional heat cure systems. See, e.g., U.S. Pat. No. 4,310,600.
Ultraviolet light is one of the most widely used types of radiation because of its relatively low cost, ease of maintenance, and low potential hazard to industrial users. Besides avoiding high temperature cure environments, rapid photo-induced polymerizations utilizing UV light for curing hard coatings offer several other significant advantages. First, the cure time is much shorter, leading to substantial economic benefits. Second, heat sensitive materials can be safely coated and cured with UV light without the use of thermal energy which could damage the substrate. Additionally, the lower levels of solvents used reduces the necessity for expensive and time-consuming pollution abatement procedures.
Although it is known that colloidal silica can be incorporated into heat cured coating systems, applicant has discovered that such colloidal silicas can be utilized to improve the abrasion resistance of ultraviolet light-curable coatings. Copending U.S. application Ser. No. 129,297, filed Mar. 11, 1980, discloses a radiation-curable hard-coating composition which requires the use of the acid hydrolysis product of an alkoxy functional silane. Copending U.S. application Ser. No. 167,622, filed July 11, 1980, now U.S. Pat. No. 4,348,462, provides UV-curable coatings which possess even greater abrasion resistance and yet do not require the use of alkoxy silanes; these results are accomplished through the specific combination of colloidal silica with acryloxy functional silanes or water miscible hydroxy acrylates, or preferably both, with catalytic amounts of UV sensitive photoinitiators.
The UV-curable coating compositions of the present invention improve upon the adhesion, abrasion resistance and resistance to yellowing on exposure to UV-radiation offered by the aforementioned compositions. In addition, the instant compositions are essentially solvent-free, and the use of highly toxic hydroxy-funtional acrylates is eliminated. Also eliminated from the instant compositions is free water, which increases shipping weight and necessitates added drying time before curing.